It is known in the manufacture of automobile frames and other tubular products to bend hollow tubes to a desired shape. One end of a tube is clamped in a clamp die. The free end of the tube is positioned adjacent a bend die around which the tube will be bent so that the bend die will define the curvature of the bend. A pressure die applies pressure to the tube to bend the tube around the bend die. When a tube is bent, basically two things happen. The tube wall on the outside of the curvature tends to collapse and thin out, and the tube wall on the inside of the tube curvature tends to compress and wrinkle.
In many applications, particularly when bending a thin wall tube or when bending around a small radius, a mandrel is inserted into the tube by a support rod before bending so that the presence of the mandrel will support the interior walls and thereby limit the amount of collapse and wrinkling encountered during the bending operation. The mandrel is often formed of ball segments that are connected together by swivel joints so that the mandrel can bend with the tube and support the tube walls, and yet the mandrel can be extracted from the bent tube after the bending.
In order to obtain a high quality bend, with minimal collapse and wrinkling, the mandrel support rod must consistently and reliably locate the mandrel at a precise location within the tube. However, it is known that during the pipe bending operation that part of the tube that is not clamped in the clamp die may become stretched axially and accordingly the bending part of the tube may be moving axially relative to the mandrel during the bending operation even as the support rod is trying to establish the mandrel at a precise location to support the tube during the bending.
Thus, it would be desirable to reliably anchor the mandrel at a predetermined location within the tube.